A variety of instruments are commonly used in modern society. Many adults, for example, will wear some type of wristwatch. Many people also will frequently wear or carry portable electronic devices, such as wireless telephones, digital music players, and personal digital assistants (PDAs). Still other types of instruments, such as pedometers, compasses, and satellite positioning devices, may be carried by individuals, e.g., during athletic activities, such as running, hiking, boating, and biking. Typically, these instruments include a mechanism, a display, and a transparent crystal or lens covering the display. With analog displays, the display often will have a dial and one or more hands that move relative to the display. Additionally or alternatively, some types of analog displays may have one or more moving dials that move below an aperture in a stationary upper dial.
The usefulness of most instruments, however, is limited by their visibility. For example, if the instrument has poor or no illumination, then the instrument cannot be viewed in low light. To improve visibility in low light, some instrument makers may include a light source, use luminescent material on the display, or both. A single light source often will illuminate only a small portion of the instrument display (e.g., that closest to the light source), however, while the use of multiple light sources typically is relatively expensive. On the other hand, most luminescent materials, such as tritium, still require some type of ambient light in order to be seen, and therefore, may not adequately function in long term, low light conditions.